The present invention relates generally to optical alignment systems, and more particularly to high precision optical alignment systems.
Electro-optical systems exist which employ active auto-alignment techniques using miniature two-axes mirror technology. For example, U.S. Pat. No. 6,020,955 (incorporated herein by reference) describes an electro-optical system including a pseudo on-gimbal automatic alignment and stabilization system. Such alignment and stabilization systems dynamically boresight and align one or more sensor input beams and a laser output beam using automatic control closed-loop feedback, a single photodetector and stabilization mirror, two off-gimbal optical reference sources and two alignment mirrors.
The alignment system includes an optical apparatus for use in auto aligning line-of-sight optical paths of at least one sensor and a laser. The optical apparatus includes at least one alignment reference source for outputting a laser reference beam that is optically aligned with the line-of-sight of the sensor, and a laser reference source for outputting a laser reference beam that is optically aligned with the line-of-site of the laser.
A laser alignment mirror adjusts the line-of-sight alignment of the laser beam, and a sensor alignment mirror adjusts the alignment of the at least one sensor. Combining optics couple the plurality of reference beams along a common optical path. A gimbal apparatus houses the photodetector which detects the plurality of reference beams. Also within the gimbal apparatus is a fine stabilization mirror, which adjusts the line-of-sight of the optical paths of the at least one sensor and the laser. A processor is coupled to the photodetector, the laser alignment mirror, the sensor alignment mirror, and the fine stabilization mirror for processing signals detected by the photodetector and outputting control signals to the respective mirrors and combining optics to align the line-of-sight optical paths of the sensor and the laser.
The alignment reference source is typically a semiconductor laser such as a laser diode. Applicants have found, however, that a disadvantage of such a configuration is that the semiconductor laser output may experience non-uniformities. For example, the semiconductor laser may dynamically change modes, or mode-hop. Mode-hopping within a semiconductor laser introduces non-uniformities in the alignment reference source. Non-uniformities in the alignment reference source can in turn degrade the accuracy of the alignment system.
Accordingly, there is a strong need in the art for an auto alignment system wherein the alignment reference source is more uniform so as, for example, not to be adversely affected by mode-hopping within the semiconductor laser.
In the light of the foregoing, one aspect of the invention relates to an optical alignment system which includes a semiconductor laser for generating an alignment beam. A photodetector detects the position of the alignment beam, wherein the photodetector provides a position feedback signal indicative of the position of the alignment beam. An optical system directs the alignment beam, wherein the optical system includes at least one beam steering device, and a controller for controlling the at least one beam steering device to control the position of the alignment beam on the photodetector based on the position feedback signal. A modulated drive current source provides a modulated drive current to the semiconductor laser, wherein the modulated drive current is modulated at a frequency and amplitude which causes the semiconductor laser to mode-hop at a frequency which is greater than the upper frequency limit of the photodetector.
A second aspect of the invention is a method for creating an optical alignment system. The method includes the step of generating an alignment beam from a semiconductor laser. A second step includes detecting the position of the alignment beam on a photodetector, wherein the photodetector provides a position feedback signal indicative of the position of the alignment beam. A third step includes using an optical system to direct the alignment beam, wherein the optical system includes at least one beam steering device, and a controller for controlling the at least one beam steering device to control the position of the alignment beam on the photodetector based on the position feedback signal. A fourth step includes providing a modulated drive current to the semiconductor laser, wherein the modulated drive current is modulated at a frequency and amplitude which causes the semiconductor laser to mode-hop at a frequency which is greater than the upper frequency limit of the photodetector.